1. Field of the Invention
This invention relates generally to diagnostic systems adapted for monitoring the operation of valves and valve operators and more particularly to non-invasive diagnostic methods and systems which are adapted for monitoring and verifying the operation of solenoid valves.
2. Description of the Prior Art
Solenoid valves or Solenoid operated valves (SOV's) generally define a class of valves which are operable through movement of an integral actuator in the form of a solenoid. This solenoid portion of the valve essentially is comprised of an electromagnet which is in the form of a coil and a movable plunger. In operation, the plunger is movable between an opened position and a corresponding closed position through application of an external voltage source upon the SOV, such as a suitable AC or DC voltage. In particular, as the voltage is applied to the SOV, a magnetic field is produced that attracts the solenoid plunger which causes a change in the state of the valve from its opened or closed position to the corresponding closed or opened position, respectively. When the voltage is thereafter removed, a spring or other force operates to return the plunger from its corresponding changed state to the original position within the SOV.
Solenoid operated valves have been widely utilized within much of industry and for many different applications. In most of these applications, the continual, proper operation of these valves is essential for maintaining effective plant operation. In particular, within the nuclear power industry, SOV's have been extensively used for providing both system operation as well as for safety related reasons. Specifically, for this type of application, proper SOV operation is especially critical in order to prevent possible release of harmful radioactive materials either directly or indirectly.
In recent years, because of numerous reported malfunctions occurring within the nuclear power industry, the INPO (Institute of Nuclear Power Operations) and the NRC (Nuclear Regulatory Commission) have instituted several rigid guidelines on that industry for verifying proper operation of solenoid valves. Particularly, nuclear power plants are now required by ASME Section XI to perform evaluation by stroke timing quarterly on all critical SOV's. Generally, the purpose for this code requirement is to detect SOV degradation through evaluation of valve position. Specifically, an SOV will take a particular amount of time to stroke or, in other words, in order to travel between its opened and closed positions, or vice-versa. In this regard, as a valve undergoes degradation through use, the stroke time of the SOV will increase. As such, the ASME code sets forth the requirement that if stroke time of an SOV should increase by 50% or more from the previous quarterly test, then the test frequency thereafter is increased to once a month until corrective action is taken.
Current inspection techniques that have been adapted for evaluating the operation of a nuclear plant's solenoid valves have proven inadequate; especially in view of ASME Section XI. In particular, in the past SOV operation had been typically verified by performing leak rate testing on the respective solenoid valves. Generally, leak rate testing procedures involve monitoring the amount of leakage from both the internal portion of the SOV to the atmosphere and also between the internal sealed parts of the device. However, contrary to the ASME requirements, leak rate testing does not provide timing of SOV operation since the valve is functionally inoperable during such testing procedures. In addition, other problems are that such procedures are rather time consuming in both setting up and testing, thus subjecting test personnel to extended periods of time within radiation areas.
Presently, there are a number of other various inspection techniques which have been utilized for verifying operation of other types of valves aside from solenoid valves. However, such diagnostic techniques are deficient for solenoid valve application. For example, some techniques involve opening or disassembly of the valve for visual inspection of the internal components. Such intrusive techniques however require additional testing following reassembly of the valve in order to verify valve operation. In addition, such techniques also do not incorporate the required stroke timing operation on the valve in accordance with the ASME code.
Other current diagnostic techniques incorporate acoustic or magnetic evaluation of the valve or, in some instances, a combination of both acoustic and magnetic evaluation techniques. Generally, acoustic techniques involve the detection of acoustic energy or vibrations produced by the internal components of the valve. For such applications, an acoustic sensor is employed in order to detect the acoustic vibrations produced by the valve. With magnetic techniques, generally the magnetic field of an installed magnet within the valve is monitored as the position of the magnet on the internal components change during operation. With this operation, a magnetic field sensor is utilized which operates to detect variations in the magnetic field strength of the moving magnet. After which, in order to determine valve operation, the data associated with either the acoustic vibrations or magnetic field strength of the particular valve is thereafter analyzed individually or in combination in order to monitor the internal valve condition.
Several problems however are known to exist with the aforementioned prior art devices. In particular, such diagnostic techniques require rather invasive procedures to accommodate valve testing. Specifically, the valve is required to be manually disassembled or otherwise modified in order to accommodate the additional equipment or components (magnets) which are necessary to accomplish the testing operation. However, as indicated earlier, such intrusive techniques require additional testing following reassembly of the valve in order to verify valve operation. In addition, when the valve is located within a radiation area, this poses an additional problem since the test personnel would be subjected to an increased radiation exposure. Furthermore, it is oftentimes a problem to interpret the data associated with such prior art devices. For example, in many instances, it is difficult to evaluate the particular time period in which the valve undergoes a change in state from either its closed or opened positions. Additionally, such techniques oftentimes do not afford consistent results from one testing operation to the succeeding test, thus not providing an accurate indication of valve operation. In particular, with SOV application, such inconsistent results could be compounded by application of the ASME code. Specifically, under code requirements, the stroke time data obtained for a particular valve can be rounded off to the nearest full second. As such, a small variation from one test to the next resulting from such inconsistent operation could potentially exceed the 50% criteria under the code, which thereafter would require an increased frequency of testing until corrective action would be taken. For example, most nuclear power plants employ small power operated valves which are capable of stroking in two seconds or less. As a consequence, a valve potentially could test at a stroke time of 1.49 seconds during one test and 1.51 seconds during the subsequent test, as a result of inconsistent testing operation. Therefore, under the code these time periods, when rounded to 1.0 and 2.0 seconds, respectively, would exceed this 50% criteria allowed by the code. In reality, however, such difference of only 0.02 seconds in stroke time is usually not an indicator of significant valve degradation requiring the increased testing procedures. However, in other circumstances, a fairly appreciable amount of valve degradation potentially could go undetected due to the inconsistencies in testing operation, which, in some instances, would result in very severe implications. In particular, as a result of testing inconsistency, the difference in stroke time detected from one test to the subsequent test potentially could be less than the actual difference in stroke time of the valve. Consequently, in these instances, valves that have undergone degradation to a point of not being sufficiently operable would continue to be used either without being changed or monitored with the increased frequency of testing.
Furthermore, many techniques also employ separate components which provide a rather cumbersome procedure for operation when in conjunction with the valve. Still these and other techniques also require additional equipment aside from that used in conjunction with the valve in order to analyze the data obtained. As such, this further delays the time required by which determination of valve operation can be made.
Because of these and other problems associated with valve inspection techniques presently employed, there now exists a strong need for an inspection technique specifically adapted for verifying operation of solenoid valves; especially those utilized within nuclear power plants.